Make it in anything you use to draw, paint, or collage images. If you want to go with bitmap format you can use software ranging from Paint (comes with Windows) and GIMP (free download) to Paint Shop Pro (cheap) to Photoshop (not so cheap). Or, you can do it in vector format with something like Inkscape or Illustrator). That said, I'm not entirely sure that the least expensive of these will export PNG format but I imagine there are free tools out there that will do the conversion for you.

There are other ways as well: for instance, you could model and render the rose in some 3D software. There's some free stuff out there - some older versions are given in the DVDs that come with some magazines - or you can pay a lot of money for something fancier.

Hope that helps - and that I haven't said anything too obvious.
M